1. Field of the Invention
This present invention relates to a laser, and more particularly to a broadband fiber laser.
2. Description of the Prior Art
In the prior art, an Erbium doped fiber laser includes a lasing cavity consisting of two reflective mirrors, and a gain medium consisting of a pumping source and an Erbium doped fiber. Hence, the light wave can resonate in the resonance cavity and will be amplified by the gain medium.
In FIG. 1, a structural diagram of the fiber laser of the prior art is shown. The fiber laser 100 is an Erbium doped fiber laser including a resonance cavity with a fiber Bragg grating (FBG) pair. The fiber laser 100 includes a pumping source 110, an optical isolator 120, a WDM coupler 130, an Erbium doped fiber 140, a fiber Bragg grating 150, a FBG 160, an application device 180, and a spectrum detection device 170. The central wavelength of the FBG 150 is identical to that of the FBG 160.
In the fiber laser 100, the pumping source 110 pumps the Erbium doped fiber 140 to spontaneously emit radiation (a light signal) within a broadband spectrum. Next, the wavelength equal to the central wavelength of the FBGs may resonate and is amplified in the gain medium between the FBGs. The fiber laser 100 outputs the passed light from the FBG 150 as a laser light.
In FIG. 2, a spectrum of the output of the fiber laser of the prior art is shown. In the spectrum of the laser light shown on the spectrum detection device 170, the wavelength is 1550.4 nm. The power is 6.0 dBm, the side mode suppression ratio (SMSR) is 56.3 dB, and the 3 dB line-width is less than 0.05 nm, limited by the optical spectrum analyzer. Wherein, the light power of the pumping source 110 is 100 mW, the length of the Erbium doped fiber 140 is 5 m, the central wavelength of the FBGs 150 and 160 is 1550.4 nm. Moreover, the reflectivity of the FBG 150 is 70%, and the reflectivity of the FBG 160 is greater than 99%.
However, the FBG 150 has to be identical to the FBG 160. Based on the condition of the FBGs, the output power of the fiber laser 100 will reduce if the central wavelength of the FBGs shifts and/or misalignment. Moreover, users have to adjust the central wavelength of the FBGs simultaneously if the FBGs are tunable, and users have to maintain the misalignment between the FBGs as less than 0.1 nm.
In FIG. 3, an Erbium doped fiber laser including a resonance cavity with an optical circulator is shown. The fiber laser 200 includes a pumping source 210, an optical isolator 220, a WDM coupler 230, an Erbium doped fiber 240, an optical circulator 250, a FBG 260, an application device 280, and a spectrum detection device 270.
The light signal can resonate between the optical circulator 250 and the FBG 260 and be amplified by the Erbium doped fiber 240, wherein the optical circulator 250 is associated with the FBG 260 to generate a linear resonance cavity.
Based on the third port of the optical circulator 250 connecting to its first port, after the second port of the optical circulator 250 receives the light signal, the light signal is coupled into the third port and the first port in turn. Next, the light signal is transmitted from the second port of the optical circulator 250 to the Erbium doped fiber 240 to be amplified again. Thus, the energy of the amplified light signal is greater than the energy of the light power provided by the pumping source 210. Finally, the fiber laser 200 outputs the passed light from the FBG 260 as a laser light.
In the case of the fiber laser 200, based on the twice light signal amplified, the light power is used and transformed effectively.
In FIG. 4, the output spectrum of the prior fiber laser is shown. In the spectrum of the laser light shown on the spectrum detection device 270, the wavelength is 1543.0 nm, the power is 12.3 dBm, the SMSR is 58.2 dB, and the 3 dB line-width is less than 0.05 nm. Wherein, the light power of the 1480 nm-pumping source 210 is 100 mW and the length of the Erbium doped fiber 240 is 3 m. Moreover, the reflectivity of the FBG 260 is 50%.
However, the optical circulator 250 wastes lots of energy of the light signal when coupling the light signal from one port to the next port. Hence, this disadvantage further influences the output presentation of the fiber laser 200.
In addition, the output power of the laser light decreases if the reflectivity of the FBG gets higher, the resonated power will be too low in the resonance cavity if the reflectivity of the FBG is low. Hence, this disadvantage means that the fiber laser cannot increase the power of the light signal and the laser light efficiently.